This is standard plate. For the license plate you pay only once - with first registration of car in province you live in. It's about 65$. LU is for my city - LUblin. Next letters and numbers are going alphabetically.
The trick is: go to the boss of local registration office and ask if they can hold you a plate with desired number. Usually they agree - especially when you tell stories about how you love your car and your wife doesn't :D
The real custom plates where you can choose all five letters and numbers costs cca. $280.